<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style type="text/css">
        .page-title {
            color: red;
            font-size: 50px;
        }
        /* 原子类 */
        .f-30px {
            font-size: 30px;
        }
        .f-50px {
            font-size: 50px;
        }
        .f-100px {
            font-size: 100px;
        }
        .f-green {
            color: green;
        }
        .f-pink {
            color: pink;
        }
        .f-blue {
            color: blue;
        }
    </style>
    <title>Document</title>
</head>
<body>
    <div class="page-title">类选择器</div>
    <h1 class="f-pink">类选择器</h1>
    <span class="f-30px f-green">类选择器</span>
    <div class="f-blue">类选择器</div>
    <p class="f-100px f-green">类选择器</p>
</body>
</html>